Senegal's election is underway, with incumbent President Abdoulaye Wade's contentious bid for a third term having sparked deadly protests in one of Africa's most stable countries.

Voting began at 8am local time and was expected to run until 6pm. Around 5.3 million people were registered to vote in Sunday's election.

Hundreds of voters booed Wade as he cast his ballot. The incumbent, accompanied by his daughter Sindiely and son Karim, arrived just after midday at the polling station in the suburb of Point E where long lines of voters had been waiting quietly for hours in the sun.

Several dozen supporters applauded at his arrival, but their appreciation was drowned out by a cacophony of boos and jeers.

Visibly angry, a tense Wade pushed one of bodyguards out of the way as he left. He beat a swift retreat after casting his ballot and did not speak to reporters.

Voting started on time in most areas in Dakar, the capital, though at some stations it was delayed by 30-40 minutes. People were turning out in numbers at the polling stations, standing in orderly queues and casting their ballots.

Senegal, a former French colony, is one of the continent's pioneer democracies, boasting an unbroken series of elections since independence in 1960.

The election is taking place amid appeals for calm by Thomas Boni Yayi, African Union president, and for "peaceful and transparent" elections by Ban Ki-moon, the UN secretary-general.

However, Wade's third-term bid is proving a test of Senegal's democratic credentials, prompting international concern after weeks of protests.

Despite having served two terms in office, a limit he himself introduced, Wade says 2008 constitutional changes extending term lengths to seven years allow him to serve two more mandates.

The second oldest African leader after Zimbabwe's Robert Mugabe, 88, Wade says he needs more time in office to finish his "grand projects".

The voting started following a proposal on Saturday night by Olusegun Obasanjo, the African Union envoy, suggesting Wade retire in two years if he is re-elected, seeking to ease tensions on the eve of the nation's contentious polls.

Opposition demands

Amsatou Sow Sidibe, one of two female candidates contesting these elections, said that her party supported Obasanjo's desire for peace, but the situation was much more complex than that.

"There are a range of reforms needed in this country, but how can you wait until the eve of an election to come and make proposals that suggest that if Wade wins the election, he could stay for two years?" she said.

"I think that is totally illegal. We totally disagree with Obasanjo's proposals."

Sidibe said that delaying the election was out of the question and it was precisely repeated election postponements that led to the standoff in Cote d'Ivoire.

"In Africa, when ever you delay an election, you are asking for trouble," Sidibe said.

The June 23 Movement, an alliance of opposition parties and activists, called for a presidential election to be organised within six to nine months, one in which Wade does not take part.

Wade is a political survivor who was in the opposition for 25 years before unseating the Socialist Party in 2000, and has remained defiant in the face of the storm of criticism at home and abroad.

He has dismissed opposition protests as "temper tantrums" and heaped derision on calls from France and the US that he retire.

He would not be dictated to by "Toubabs", he said, using the Wolof term for Westerners.

While there is no front runner opposition candidate, opinion polls suggest that former prime ministers Macky Sall and Idrissa Seck are tipped to be his strongest competitors.

Sall received a warm reception when he voted in his hometown of Fatick, 160km east of Dakar on Sunday.

Senegal's music icon and political activist Youssou N'Dour called for real change in the country, warning its citizens would not accept an electoral coup.

The Grammy-winning singer whose own ambitions to run in the election were rejected by the country's highest court, voted in the suburb of Mermoz, walking with a slight limp after being injured during a protest.

"What counts, what is important to the Senegalese, is change. Real change," N'Dour said.

"We will not accept an electoral coup ... the Senegalese will not accept that these elections be sullied," he said.

Despite its stability, Senegal a nation of 13 million whose main earners are fishing, tourism and groundnut production has a large proportion of people living below the poverty threshold.

Those who support Wade point to considerable infrastructure development under his rule.

However, his opponents argue that his focus on grand legacy projects has left him out of touch with the concerns of the average Senegalese.

Experts say that while the opposition has battled to unify around a candidate and therefore significantly undermining their chances of defeating Wade, there will an opportunity for the opposition to rally around one candidate if they manage to prevent the incumbent from winning a first round victory.